1. Field of the Invention
The present invention relates to an improvement of a synchronous separation circuit used in a television video circuit, more particularly, it relates to a synchronous separation circuit enabling the separation of a horizontal synchronizing signal and a vertical synchronizing signal from a composite synchronizing signal, based on a digital signal processing technique.
2. Description of the Related Art
In general, a raster scanning method is widely employed in the television video circuit, and in this method, an image is separated by a plurality of scanning lines and each of the scanning lines is processed in a time-series. As is well known, there are two types of raster scanning methods; a non-interlace scanning type and an interlace scanning type, and in both types, the synchronizing signal must be used for scanning the image as a reference signal. Note, a horizontal synchronizing signal is used as the scanning reference signal for the horizontal direction and a vertical synchronizing signal is used as the scanning reference signal for the vertical direction.
The synchronous separation circuit is usually located after a video amplification stage in the television video circuit, and is used for separating the composite synchronizing signal from a composite video signal, and for further separating the horizontal synchronizing signal and the vertical synchronizing signal from the composite synchronizing signal.
The horizontal synchronizing signal and the vertical synchronizing signal are input to a deflection circuit having horizontal deflection and vertical deflection circuits, and the output of the deflection circuit is sent to horizontal and vertical deflection coils.
Conventionally, a differentiating circuit is provided for obtaining the horizontal synchronizing signal and an integrating circuit is provided for obtaining the vertical synchronizing signal from the composite synchronizing signal. These differentiating and integrating circuits are usually constituted by resistors and capacitors, and the horizontal and vertical synchronizing signals are obtained from the composite synchronizing signal through the resistors and capacitors based on an analogue signal processing technique. Namely, the differentiating circuit is a high-pass filter and the integrating circuit is a low-pass filter.
Problems occur, however, in the analogue signal processing techniques used in these circuits, as explained in detail hereinafter.